1. Field of the Invention
The present invention relates to a wire-wound type chip coil in which two wires are wound around a core and to a winding method therefor.
2. Description of the Related Art
A common-mode choke coil disclosed in Japanese Unexamined Patent Application Publication No. 2006-121013 is an example of a common-mode choke coil according to the related art. This common-mode choke coil has a configuration in which, after a first wire is wound around a winding core section and both of its ends are fixed on flange sections at both ends of the winding core section, a second wire is wound over the first wire and both of its ends are fixed on the flange sections.
With this configuration, a differential signal on a differential-transmission line is transmitted and an intruding common-mode noise is removed.
Another coil unit according to the related art is disclosed in Japanese Unexamined Patent Application Publication No. 2005-166935. Unlike Japanese Unexamined Patent Application Publication No. 2006-121013, this wire-wound type coil is formed by simultaneously winding a first wire and a second wire in pairs around a winding core section.
However, the above-described related-art wire-wound type coils have the problems described below.
FIG. 13 is a schematic cross-sectional view to describe a problem in the wire-wound type coil according to Japanese Unexamined Patent Application Publication No. 2006-121013. FIG. 14 is a schematic cross-sectional view to describe a problem existing in the wire-wound type coil according to Japanese Unexamined Patent Application Publication No. 2005-166935. A first wire is represented by a black circle, and a second wire is represented by a white circle. In FIG. 14, the numeral in each of the black and white circles represents the number of the turn of the wire corresponding to the circle.
For a wire-wound type coil 100 as shown in FIG. 13, after a first wire 121 is directly wound around a winding core section 111 of a core 110 from the side adjacent to a flange section 112 toward the side adjacent to a flange section 113, as indicated by the black circles in FIG. 13, a second wire 122 is wound over the first wire 121, as indicated by the white circles in FIG. 13. Because the same winding operation must be performed twice, the productivity is reduced.
Furthermore, because the second wire 122 is wound on top of the first wire 121, the wire-wound type coil 100 has a winding structure in which a wire portion 122-1 of the first turn of the second wire 122 is disposed between a wire portion 121-1 of the first turn of the first wire 121 and a wire portion 121-2 of the second turn of the first wire 121. Thus, a wire portion 122-n of the second wire 122 of the last turn is not disposed on top of the first wire 121 and has to be wound directly around the winding core section 111. That is, the wire portion 122-n of the last turn of the second wire 122 suffers from a phenomenon in which it is arranged below a layer in which it should be disposed (hereinafter, this phenomenon is referred to as a layer-down). If such a layer-down occurs, when a predetermined differential signal is input, the noise power ratio for output noise may be increased, such that the noise reducing effect is decreased.
Moreover, such a layer-down must sacrifice the number of turns of the coil by the amount of the last turn. Thus, the number of turns cannot be sufficiently increased, and the range of an obtainable inductance value is reduced.
For a wire-wound type coil 150 as illustrated in FIG. 14, after a first wire 121 and a second wire 122 are wound side by side around a winding core section 111, the first wire 121 and the second wire 122 are simultaneously wound such that the second turn of the first wire 121 is arranged downstream of the first turn of the second wire 122 and such that the second turn of the second wire 122 is disposed on top of the first turn of the first wire 121 and that of the second wire 122. The third and subsequent turns are also wound similarly to the second turn.
Accordingly, with such a winding method, the distance d between the first wire 121 and the second wire 122 at the second and subsequent turns is greater than necessary, such that the wire-wound type coil 150 is unbalanced as a coil. Because of this, the magnetic coupling between the first and second wires 121 and 122 may be decreased, mode conversion into noise may occur, and a noise reducing effect may not be obtainable.